This invention relates generally to solid state laser oscillators and amplifiers and more particularly, to laserable rods with flanged end-caps where at least the most distal sections are undoped.
Many potential applications motivate the development of efficient, compact 1 .mu.m laser systems with operational lifetimes capable of exceeding thousands of hours. Yb-doped laser hosts offer spectroscopic and laser properties that make them promising candidates for high power 1 .mu.m laser systems. In particular, Yb:YAG has a long storage lifetime of 951 .mu.s and a very low quantum defect resulting in less heat generation during lasing than comparable Nd based systems. In addition, the broad pump line at 940 nm makes this material highly suitable for diode pumping using InGaAs based diode-pumped lasers. InGaAs based lasers are more robust than AlGaAs diode-pumped lasers which are used to excite Nd:YAG at approximately 808 nm. Furthermore, because the 940 nm absorption feature in Yb:YAG is approximately 10 times broader than the 808 nm absorption feature in Nd:YAG, the Yb:YAG system is less sensitive to diode wavelength specifications.
A common problem with lasers in general, and with solid state lasers in particular, is the temperature rise in the laserable medium. This effect is the result of only a percentage of the applied pump energy being converted into useful laser radiation while at least a portion of the remaining energy is transferred into competing mechanisms such as radiationless transitions and absorption by the host lattice. To counteract the thermal effects generated by the pump radiation, typically the laserable medium must be cooled.
Cooling the entire laserable medium of a solid state laser presents a difficult problem. First, the cooling fixture should allow as much of the laserable medium to be cooled as possible. Typically some portion of the laserable medium must reside outside of the cooling fixture in order to create a leak-free joint between the fixture and the medium. Second, the cooling fixture should not interfere with the delivery of pump radiation to the laserable medium. Lastly, even if the entire length of the laserable medium is cooled, the optical coatings on the uncooled end surfaces are not cooled and are therefore subject to degradation due to temperature effects, an effect which is aggravated by the high power density of the incident pump radiation.
Some of the effects noted above which result from non-uniform cooling are further exacerbated in end-pumped laser systems. In this type of laser a lens duct is preferably used to homogenize, focus, and concentrate the radiation from the pump source into the laser rod. The end of the laser rod which is coupled to the lens duct is uncooled. Unfortunately, given the high intensity pump radiation passing through the end face of the laser rod and assuming that the laser rod is doped throughout with lasing ions, the ends of the laser rod can experience a substantial rise in temperature during lasing. This rise in temperature can negatively impact the performance of the laser by accelerating the degradation in the optical coatings, increasing the ground state re-absorption losses in quasi-three-level lasers, and eventually leading to the catastrophic failure of the laser due to thermally induced stress fractures.
Several approaches have been attempted to overcome the cooling problems associated with lasers. G. E. Devlin et al. (Applied Optics, Vol. 1 (1) 11 (1962)) recognized the benefits of laser rods consisting of a cylindrical lasing core clad with a laser-inactive concentric sleeve. It appears, however, that these benefits were never realized with crystalline laser-active media due to the exceedingly difficult task of producing concentric cylinders of composite crystalline laser rods. Although a few concentric rods with a ruby core and sapphire cladding may have been fabricated using a fusion or melt cladding process, to our knowledge no such composite has been described in a scientific journal. These fabrication techniques pose severe limitations on the types of achievable configurations. Furthermore, the interface region between the laser-inactive cladding and the laser-active core is about a millimeter thick and exhibits striations which puts the lasing core under stress and adversely affects beam quality. It is also limited to components with round cross sections because the cylindrical lasing medium rotates during the crystal growing process. This type of structure is necessarily limited to core diameters which are larger than about 1-2 millimeters.
U.S. Pat. No. 5,563,899, entitled "Composite Solid State Lasers of Improved Efficiency and Beam Quality," discloses composite lasers formed of one or more sections of non-lasing material diffusion bonded to one or more sections of lasing medium. If the non-lasing sections are bonded at the ends of the lasing medium, the entire lasing medium can be cooled thereby minimizing thermal problems. A continuation-in-part application of this patent, application Ser. No. 08/713,436, now U.S. Pat. No. 5,852,622 was filed on Sep. 11, 1996 by H. E. Meissner et al. H. E. Meissner is a common inventor with the present application. Other composite laser configurations are disclosed in pending application Ser. No. 08/580,730, now U.S. Pat. No. 5,846,638, filed Dec. 29, 1995 by the same H. E. Meissner. The bonding techniques utilized in the above-cited applications are described in U.S. Pat. No. 5,441,803, as well as pending applications Serial Nos. 08/330,174 now abandoned and 08/569,761, now abandoned all naming H. E. Meissner as the inventor.
From the foregoing, it is apparent that an improved laser rod configuration allowing more efficient cooling of the laserable medium as well as enhanced coupling of pump energy is desirable.